A variety of rotary seals are used in gas turbine engines which are generally of the type including rotating members cooperating across a narrow gap with another member which is relatively stationary. Such seals are used, in some cases, between stationary members and a rotating shaft or drum for the purpose of maintaining different pressures in chambers on each side of the seal. For example, in one type of gas turbine engine, a plurality of rows of rotor blades extend radially outward from a rotor shaft across a flow path for the working medium gases. Collaterally, a plurality of rows of stator vanes extend radially inward across the flow path from a stator case or shroud. In some types, the stator vanes are cantilevered inward from the stator case. The vanes are positioned to direct the working gases toward or away from the adjacent rotor blades. A stator has a sealing surface circumscribing the tips of blades in each row of blades and, in the cantilever stator vane types, the rotor is provided with a sealing surface circumscribing the tips of the stator vanes in each stator vane row.
As the clearance between the tips of the blades or vanes in each row and the corresponding sealing surface is increased, substantial amounts of working medium gas escape circumferentially over the tips of the blades and/or stators, reducing the aerodynamic efficiency. Furthermore, when the clearance is increased, additional amounts of working medium gases leak axially over the tips from the downstream end to the upstream end of the blades or rotors. Therefore it is desirable to keep the clearance to a minimum. However, it is also necessary to accommodate various dimensional changes which occur during initial startup, thermal excursions, high G turns, etc. Generally, there is some wear-in of the parts under these conditions, particularly during engine startup.
It is known that the more desirable condition is for the tips or knife edges to cut grooves into the corresponding sealing surface rather than for the tips or knife edges to sustain wear. U.S. Pat. Nos. 4,238,170 and 4,239,452 provide the sealing surface of the stator or shroud with interior circumferential grooves circumscribing the tips of the blades, but this arrangement presents alignment difficulties while failing to accommodate thermally-induced axial displacement of the blades relative to the stator or shroud.
A variety of rotary seal arrangements have been disclosed in the literature in which rotating members generate, cut or abrade a path in a softer, e.g., abradable, cooperating member such as filled honeycomb, porous metal, friable ceramic or the like. In some of these arrangements, it has been found that inadequate sealing or seizing of the cooperating members can result. In other such arrangements, local "hot-spots" and burning of non-abrading members can result. Examples of seals utilizing an abradable member are disclosed in U.S. Pat. Nos. 3,068,016; 3,481,715; 3,519,282; 3,817,719; 3,843,278; 3,918,925; 3,964,877; 3,975,165; 4,377,371 and 4,540,336. The abradable seal is adapted to flake off or abrade, when there is a thermal transient or shock loading causing the blade tip to strike the seal. U.S. Pat. No. 4,377,371 points out that certain materials used as abradable seals are vulnerable to large scale spalling propagated by the presence of cracks in the seal surface and discloses the glazing of the seal surface by the use of a laser beam to produce a fine micro-crack network in the seal surface. In a paper "Development of Improved-Durability Plasma Sprayed Ceramic Coatings for Gas Turbine Engines", presented by I. E. Sumner and D. Ruckle at the AIAA/SAE/ASME 16th Joint Propulsion Conference, AIAA-80-1193, segmented laser scanned coatings were reported to perform poorly.
British Patents 853,314 and 1,008,526 disclose turbine or compressor blades formed with ribs on their tips for providing a seal with a rotor or stator shroud, the ribs or cooperating seal surface being removable when worn out. U.S. Pat. No. 4,148,494 discloses a gas turbine blade or vane having an abrasive tip comprising an electrodeposited matrix of nickel or an alloy containing nickel in which are entrapped abrasive particles, e.g., borazon particles, protruding from the tip. Abrasive tips of the type described in this patent are difficult to produce and are extremely expensive. U.S. Pat. No. 3,339,933 discloses blade teeth coated with bonded alumina which expand into cooperating honeycomb members to form a seal. U.S. Pat. No. 3,537,713 discloses a rotating sleeve having inwardly projecting teeth coated with a hard protective material, such as a molybdenum or nickel aluminide, which displaces a rub-resistant material on a stationary cooperating member to form alternate ridges and grooves.
U.S. Pat. No. 4,884,820 discloses the laser treatment of the tip of a blade that was coated with a ceramic or metallic carbide coating to form a plurality of laser-formed depressions that produce a wear-resistant, cutting surface capable of cutting into an abradable material in a rotary gas seal.
Although the above coatings provide a good wear-resistant, cutting surface for blades, the coatings sometimes do not adhere sufficiently to the blades to remain bonded to the blades when used in high temperature environments. For example, although ceramic coatings such as alumina-based coatings, provide good wear-resistant, cutting surfaces for many substrates, they do have poor bond strength to substrates such as titanium alloys. However, if the substrate, such as a titanium alloy, is roughened by grit blasting with an abrasive material, such as alumina grit, then a good bond can be obtained for the ceramic coatings. Unfortunately, the grit blast operation will generally cause a large fatigue debit to the substrate. Fatigue is the progressive phenomenon of failure that occurs in materials when they are subjected to cyclic loading at stresses having a maximum value less than the tensile strength of the materials. Fatigue can generally culminate in fracture after a sufficient number of cyclic loadings. Since fatigue causes materials to fail sooner and/or at lower loads than would be expected, its net effect has been to either shorten the useful life period of materials at the same load or reduce the allowable load for the same life period. Thus the use of grit blast is not desired since it can cause fatigue debit in the substrate.
U.S. Pat. No. 4,826,734 discloses substrates coated with tungsten carbide-cobalt coatings that have a strain-to-fracture of greater than 4.3.times.10.sup.-3 inch per inch. However, although it will provide an adequate wear-resistant and cutting surface, the ceramic coatings generally provide a more desirable wear-resistant cutting surface for use in the tip of blades for compressors.
It is an object of the present invention to provide a duplex coating for substrates that has good fatigue characteristics while also providing a good wear-resistant cutting surface.
It is another object of the present invention to provide a duplex coating for substrates such as titanium that comprises an undercoat of tungsten carbide-cobalt and a top coat of a ceramic coating.
It is another object of the present invention to provide a duplex coating for titanium based substrates comprising an undercoat of tungsten carbide-cobalt having a strain-to-fracture greater than 4.3.times.10.sup.-3 inch per inch and a top coat of an alumina-based coating.
The foregoing and additional objects will become more apparent from the description and disclosure hereinafter.